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Date: Tue, 29 Jul 2003 12:35:57 -0400
From: Hyman Rosen <hyrosen@mail.com>
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Subject: Re: Non-philosophical definition of Eiffel?
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Date: 2003-07-29T12:35:57-04:00
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Dmitry A. Kazakov wrote:
> There is little sense in contravariant arguments

OK, I just want to make sure that we're talking about
the same thing, so let me give specific examples. I'll
write in C++ terms, just because.

     struct B1 { };
     struct D1 : B1 { };

     struct B2
     {
         virtual B1 &f1();
         virtual B1 &f2();
         virtual void f3(B1 &);
         virtual void f4(D1 &);
         virtual void f5(B1 &);
     };

     struct D2 : B2
     {
         B1 &f1();
         D1 &f2();
         void f3(B1 &);
         void f4(B1 &);
         void f5(D1 &);
     };

     D2 d2;
     B2 *b2 = &d2;

f1) This is ordinary overriding, with same return type.
     Pretty much legal in every language.
f2) This is covariant return type. Legal in C++. This
     preserves LSP, since a call b2->f2() must return a
     B1 &, and a D1 & is a B1 &.
f3) Again, ordinary overriding, with the same argument
     types in base and derived. Legal everywhere.
f4) Contravariant argument types. This also preserves
     LSP, but I don't know of a language which implements
     it. LSP is preserved because a call of b2->f4() can
     only pass a D1 & argument, and a D1 & is a B1 &.
f5) Covariant argument types. This is Eiffel's model.
     LSP is not preserved, because a call to b2->f5()
     may pass any B1 &, while d2.f5() wants only a D1 &.